The present invention relates to energy generation systems and, more particularly, to a forced-circulation dual-phase reactor. A major objective of the present invention is to provide for enhanced core neutron power stability during pump shutdowns in a forced-circulation boiling-water reactor (FCBWR).
Dual-phase reactors store heat generated by the core primarily in the form of vapor pressure generated by the vaporizing of a liquid heat transfer medium. The vapor pressure can be used to rotate a turbine that drives a power output generator to produce electricity. In a dual-phase reactor, energy from the core vaporizes liquid so that some of the heat energy generated by the core is stored in the form of the phase conversion from liquid to vapor. The predominant type of dual-phase reactor is the boiling-water reactor (BWR). Accordingly, much of the following discussion concerning BWRs is readily extrapolated to other dual-phase reactors. In a single-phase reactor, the fluid remains liquid, and the energy generated by the core is stored primarily in the form of elevated temperatures. Liquid-metal reactors define one type of single-phase reactor.
In a BWR, heat generated by nuclear fission in a core can be used to boil water to produce steam. Water passing through the core without being vaporized is recirculated within a reactor vessel to provide a continuous flow of water through the core. The steam that is generated can be separated from the water and transferred from the reactor vessel to deliver energy. For example, the steam can be used to drive a turbine, which in turn can be used to drive a generator to produce electricity. In the process, the steam condenses and can be returned to the vessel. The condensate is merged with the internally recirculated water and continues to aid heat transfer.
BWRs can be distinguished by the means employed to recirculate the water within the reactor vessel. Forced-circulation boiling-water reactors (FCBWRs) rely primarily on pumps to drive the water along a recirculation path. Natural-circulation boiling-water reactors (NCBWRs) rely primarily on the driving force provided by the density difference between the downcomer and the steam column above the core. NCBWRs have the advantage of simplicity. However, their inherently lower pumping capacity limits reactor power output. Accordingly, the largest capacity BWRs are all FCBWRs. The distinction between FCBWRs and NCBWRs notwithstanding, FCBWRs are preferably designed to take advantage of natural circulation to allow decay heat to be removed from the core in the event the pumps are shut down.
In both types of BWRs, the core is immersed in water. Water flowing up through the core is partially converted to steam. To promote natural circulation, water rising up from the core is guided vertically to promote steam-water separation and to support a relatively low-density steam/water head above the core. Water recirculates down a downcomer annulus between the reactor vessel and the chimney and core. The water in the downcomer is denser than the steam and water mixture in the core and chimney region. The difference in density forces circulation up through the core and chimney and down through the downcomer.
One reason natural-circulation provides for limited power output is that its limited circulation rates can provide more time than is optimal for the water flowing through the core to be converted to steam. The excess boiling results in a larger volume of steam in the core. This larger steam volume adversely affects core stability, as the stability-decay ratio of the nuclear fission rate is dependent on the ratio of two-phase pressure drop to single-phase pressure drop. In ratio of two-phase pressure drop to single-phase pressure drop. In NCBWRs, this problem is addressed by limiting the amount of heat generated by the core, and thus the power output of the reactor.
FCBWRs, on the other hand, are typically designed so that they exceed the power output that would be available using natural circulation alone. Total pumping power failure in an FCBWR operating at full capacity could result in excess boiling and core instability. This scenario is addressed by providing several independent pumps so that the likelihood of total pumping power failure is minimized.
Despite the levels of safety afforded by redundant pumping, there is still value in enhancing the throughput due to natural circulation in an FCBWR. Natural circulation is especially attractive as a safety backup due to its independence from active components. Thus, improvements in natural circulation are highly desirable in the context of FCBWRs.